(1) Field of the Invention
The present invention relates generally to surveillance systems, and in particular to a surveillance system used in conjunction with a device to provide location and identification of friendly personnel or vehicle targets.
(2) Description of Related Art
Surveillance systems for the detection of personnel and vehicles within sensitive or secured areas are being deployed in significant numbers due to increased concern for security and also due to the availability of effective, affordable sensors. These sensors may use detection methods, and may operate over distances and in environmental conditions that make it impossible for the surveillance system to achieve a detailed identification of each detected target. A “friendly” target is defined as an entity that is authorized to conduct activities within the area. “Unfriendly” targets are not authorized to be in the area and thus are assumed to be hostile. A way must be provided for target Identification—Friend or Foe (IFF) to differentiate between these two classes of targets.
One prior art method of determining the identity of friendly airborne targets is to couple to the surveillance radar antenna a separate antenna that transmits an interrogation signal into the same volume being searched by the radar. A cooperating, friendly aircraft carries on board a transponder that receives the interrogation signal and, after a small delay, responds with a coded reply. The reply may contain specific aircraft identification and altitude or other information. The time delay between sending the interrogation signal and receipt of the reply is used to determine the range to the friendly target. The target identification is then correlated with the skin return detected by the radar, if such detection has occurred. The interrogation signal and transponder reply are typically generated at frequencies that are much different from the radar operating frequency. An aircraft detected by the surveillance radar that does not respond to the interrogation with a proper reply can be labeled a “foe” target. Other prior art IFF techniques have used the surveillance radar signal as the interrogation signal with an appropriate transponder reply on a different frequency. In each of these cases the interrogation and reply signals are typically produced in the microwave region of the radio frequency spectrum, and the transponder must be capable of generating a reply of sufficient microwave power to be received at the radar site. These prior art methods are not practical for use in a surveillance system designed to detect personnel and vehicles within sensitive areas having typical dimensions of no more than a few kilometers.
A prior art example of a data communication network used to provide location information of a multiplicity of friendly entities as well as an IFF function is provided by U.S. Pat. No. 6,181,272 to Kirk. This patent describes a method using a central control station that transmits position requests to one or more field agents. Each of the field agents determine their position by use of a global positioning system (GPS) receiver and reply to the position request with a message that may include location, identification and a code. The central control station includes a receiver to collect the incoming data, a computer and a display to provide a graphical indication of the locations of all participating agents. When it is desired to confirm the “friendly” status of any replying agent, a laser designator is directed to the location of the agent. Each agent carries a laser detector that causes an encrypted response to be sent back to the central control station. This data communication network can determine the GPS coordinates of all participating agents, but the location accuracy of the replying agent is limited to the accuracy of a GPS receiver operating autonomously.
These prior art examples have several limitations if an attempt is made to apply them for both detection of unauthorized intruders and identification of authorized entities within a secured area having dimensions of no more than a few kilometers. The radar and IFF system for airborne applications usually requires a separate interrogation transmitter and microwave receiver at the radar site to stimulate and receive responses from IFF equipped aircraft. The IFF units are not easily reconfigured as man carried, low power consumption devices that typically operate at a range in the order of a kilometer from the surveillance radar. The method taught by the Kirk patent requires a laser interrogator for positive identification of participating agents. It is capable of determining the location of participating agents only, and has no capability to detect hostile intruders. Further, it does not present a display of the relative position of the responding agents with respect to various physical structures, etc. within the area unless a map overlay is added to the display. For these and other reasons, a method or a means of target Identification—Friend or Foe (IFF) is needed that is compatible with surveillance sensors and systems used for security purposes.